1. Field of the Invention
The present invention relates to an optical recording medium, which is recorded, erased and reproduced by irradiating a light beam, particularly, relates to a rewritable phase change type optical recording medium such as an optical disc and an optical card, which exhibits higher sensitivity even at higher linear velocity and enables an excellent recording to realize.
2. Description of the Related Art
The conventional arts related to a rewritable phase change type optical recording medium are as follows: such a rewritable phase change type optical recording medium is formed with an amorphous record mark through the processes of melting a recording layer by impressing a laser beam pulse on the recording layer while recording and rapidly cooling down the melted recording layer. A reflectivity of the record mark is lower than that of the recording layer in crystalloid and the record mark is optically read out as a recorded information. When erasing the record mark, the record mark in amorphous is transformed into crystalloid by adjusting temperature of the recording layer within the range of more than crystallizing temperature and below the melting point by irradiating a laser beam in a lower power than that for recording.
With respect to a material enabling such the recording method, alloys such as Gexe2x80x94Sbxe2x80x94Te alloy and Agxe2x80x94Inxe2x80x94Sbxe2x80x94Te alloy are preferably utilized due to a relatively higher crystallizing speed. In an optical recording medium having a recording layer formed by using such the recording material as the alloy mentioned above, a dielectric layer having heat-resistivity and transparency is generally formed on both sides of the recording layer in order to prevent the recording layer from deforming and opening.
Further, if the dielectric layer is formed by a material containing ZnS component, there existed an optical recording medium having an interfacial layer such as nitride inserted between the recording layer and the dielectric layer on one side or both sides of the recording layer in order to prevent the recording layer from the S (sulfur) penetrating. Furthermore, an art such as laminating a metallic reflection layer consisting Al or Ag as a major component on the surface of the dielectric layer opposite to the incident direction of laser beam is well known with respect to enabling higher reflectivity.
Problems arisen along with the above-mentioned phase change type optical recording medium are as follows: in the conventional disc structure, only a protective layer being composed of a dielectric layer is provided between a recording layer and a reflection layer. The protective layer fulfils only a function of preserving or transmitting heat generated while recording, so that the heat is hard to be controlled. In other words, by thickening the protective layer provided between the recording layer and the reflection layer, the heat tends to be preserved. Consequently, a melting area of the recording layer enlarges and a record mark in amorphous becomes wider, and then a modulation factor can be increased. However, on the contrary, the heat spreads through a crystalline area and forms microscopic amorphous therein when the melting area enlarged. Accordingly, a reflectivity is deteriorated. On the other hand, by thinning the protective layer provided between the recording layer and the reflection layer, a reflectivity increases. However, a modulation factor is deteriorated.
As mentioned above, it is hard to improve both the modulation factor and reflectivity simultaneously by only one layer of the protective layer provided between the recording layer and the reflection layer, wherein a number of layers becomes two if an interfacial layer being adjacent to the recording layer and provided on the side facing toward the reflection layer is counted. Further, if the protective layer is thin, the modulation factor becomes higher by increasing a laser beam power. However, a lower laser beam power is desirable in consideration of lifetime of a laser device in hardware. Furthermore, in a case of recording by higher linear velocity, a higher laser beam power is required for recording according to the principle of phase change. In some cases, the required laser beam power may exceed the upper limit of a recording and reproducing apparatus for personal use.
Maintaining a certain reflectivity and simultaneously obtaining a higher modulation factor while lowering a laser beam power has been the subject of the prior art. As a method for obtaining a higher modulation factor, that is, as a method for increasing a recording sensitivity, there existed the prior art. The Japanese Patent Laid-open Publication No. 61-272190/1986 discloses that a thin film of Ge, Cr, Ti and Ni, which has a higher absorption index for the wavelength range of a semiconductor laser beam, is inserted. However, the thin film is directly contacted with a recording layer, so that the thin film is molten with the recording layer and the molten thin film and recording layer compose a different component from the original ones. Consequently, a characteristic of recording is deteriorated. Actually, a jitter and a reflectivity was extremely deteriorated according to the feasibility study by producing a sample of Agxe2x80x94Inxe2x80x94Sbxe2x80x94Te alloy as a recording layer component.
The other Japanese Patent Laid-open Publications, mentioned below, disclose that reducing a difference of light absorption between an amorphous section and a crystalloid section by inserting a layer having a function of absorbing light and consequently improving the recording sensitivity in the crystalloid section can improve a jitter characteristic although a total recording sensitivity is not aimed to be improved.
According to the Japanese Patent Laid-open Publication No. 11-66611/1999, the optical recording medium is composed of a first dielectric layer, a recording layer, a second dielectric layer, a reflection layer, a third dielectric layer and a protective film. They are laminated on a substrate in order as a layering structure. However, since the reflection layer is laminated by an element consisting Si or Ge having a function of absorbing light as a major component in some tens nm thick, a sufficiently high reflectivity can not be obtained in comparison with the reflection layer composed of an alloy containing Al or Ag.
According to the Japanese Patent Laid-open Publication 2000-182277/2000, the optical recording medium is composed of a first dielectric layer, a recording layer, an absorbing amount compensating layer having a light absorbing functional layer and a reflection layer. They are laminated on a substrate in order as a layering structure. However, since the absorbing amount compensating layer is directly contacted with the reflection layer, heat generated in the absorbing amount compensating layer flows through the reflection layer and fails to be transmitted to the recording layer although a light absorbing amount can be controlled. Accordingly, the optical recording medium failed to be a recording medium in high sensitivity because the heat was not transmitted to the recording layer.
Accordingly, in consideration of the above-mentioned problems of the prior arts, an object of the present invention is to provide an optical recording medium, which can improve a recording sensitivity in a recording layer without deteriorating characteristics such as a reflectivity and a jitter since a reflectivity in a reflection layer and a light modulation-factor in a phase change state between crystalline and amorphous in a recording layer is always maintained high. Further, the optical recording medium can obtain a higher light modulation factor even though a laser beam in a lower power is irradiated. Furthermore, the optical recording medium can maintain a higher recording characteristic for a recording by a higher linear velocity.
In order to achieve the above object, the present invention provides, according to an aspect thereof, an optical recording medium comprising a substrate, a first protective layer, a recording layer, a second protective layer, a light absorbing and heat generating layer, a third protective layer and a reflection layer laminated in order, wherein the light absorbing and heat generating layer is one of a metal layer and an alloy layer containing more than one element out of Ge, Sn, Pb, Cr, Ti, In, Si, Cd, Se, W, Mo, Zr, Nb, Zn and Hf.
According to another aspect of the present invention, there provided an optical recording medium comprising a substrate, a first protective layer, a recording layer, a second protective layer, a light absorbing and heat generating layer, a third protective layer and a reflection layer laminated in order, with defining that: a second heat conductive distance between the light absorbing and heat generating layer and the recording layer is d2, wherein heat is transmitted from the light absorbing and heat generating layer to the recording layer through the second protective layer; and a third heat conductive distance between the light absorbing and heat generating layer and the reflection layer is d3, wherein heat is transmitted from the light absorbing and heat generating layer to the reflection layer through the third protective layer, the optical recording medium is further characterized in that a relation between d2 and d3 is d2xe2x89xa6d3 less than 30xc3x9710xe2x88x929 [m], wherein the second heat conductive distance d2 and the third heat conductive distance d3 are d2=(t2/"sgr"2) [m] and d3=(t3/"sgr"3) [m] respectively, where t2 is a thickness of the second protective layer, t3 is a thickness of the third protective layer, "sgr"2 is a heat conductivity of the second protective layer, "sgr"3 is a heat conductivity of the third protective layer, and wherein "sgr"2 and "sgr"3 are expressed in a value relative to a heat conductivity "sgr"1 of ZnS(80)xe2x80x94SiO2 (20) being 1.0.
Other object and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawing.